Comparison of Two Bone Surrogates for Interbody Device Subsidence Testing

Bone surrogates are proposed alternatives to human cadaveric vertebrae for assessing interbody device subsidence. Polyurethane foam blocks are an accepted surrogate for cancellous bone but do not share their heterogeneous bone density distribution. Synthetic vertebrae have been recently developed as an alternative bone surrogate with representations of cortices, endplates, and cancellous bone. The efficacy of each surrogate was evaluated by uniaxially indenting it with an interbody device. The force-displacement curve profiles, failure forces, and depth of implant subsidence were compared for devices seated centrally and peripherally on the surrogates. The synthetic endplate mimicked human endplates through a gradually increasing endplate thickness toward the periphery. This enabled the synthetic vertebrae to provide additional subsidence resistance to implants seated at the periphery. By contrast, the foam block was insensitive to implant placement. Absence of failure in synthetic vertebrae from peripheral implant indentation suggests the synthetic endplate is stronger than human endplates but further study with human cadaveric vertebrae is needed.